<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title></title>
    <style media="screen">
    * {margin:0; padding:0; list-style:none}
    #ul1 {display:flex;}
    #ul1 li {flex:1; border:1px solid black;}
    </style>
  </head>
  <body>
    <ul id="ul1">
      <li style="min-width:400px;">1</li>
      <li>2</li>
    </ul>
  </body>
</html>
